In certain video disc systems, information is recorded as geometric variations within spiral or circular information tracks in the surface of a disc record. A signal pickup stylus engages the track and cooperating with signal pickup circuitry, recovers the recorded information as an electrical manifestation thereof. These recovery systems may be of the capacitive type as described in U.S. Pat. No. 3,842,194 Clemens or the pressure sensitive type as indicated in U.S. Pat. No. 4,037,253--Nagaoka. In either system, the information tracks and the recorded information therein is in an extremely high density format in order to engraft an acceptable amount of information on a single record. The resultant spatial dimensions of both the tracks and the geometric variations constituting signal within the tracks are in the order of micrometers and tenths of micrometers, respectively.
During signal recovery, relative velocity is created between the signal pickup stylus and the signal variations, i.e., the disc record, by rotating the disc at a prescribed velocity. Variations from the prescribed or desired stylus-disc relative velocity result in undesired distortion in the recovered signal. Velocity variations as small as 0.01 percent will noticeably affect video picture quality, which variations typically originate from track eccentricity, disc warp, disc pressing distortions, recording errors, turntable eccentricities, etc. Due to the relatively high frequency of velocity deviations and relatively large mass and high speed of rotation, e.g., 450 RPM, of the player turntable, it is not feasible to correct for stylus-disc velocity variations by actively controlling the turntable speed. Velocity correction has therefore been performed either electrically as in U.S. Pat. No. 4,150,395--Pritchard or by translating the stylus along the information track by means of a servo system, see for example, U.S. Pat. No. 3,711,641--Palmer.
In the foregoing Palmer type system, the signal pickup stylus is secured to a stylus arm arranged substantially tangent to the track being currently engaged. A second end of the stylus arm is connected to an electromechanical transducer for producing longitudinal motion in the stylus arm and thus, a translation in the stylus arm along the track. The transducer is responsive to signal derived from the signal recovered from the record and indicative of relative stylus-disc velocity errors. This signal conditions the transducer to move the stylus/stylus arm in the direction of disc rotation to reduce the relative disc-stylus velocity and move the arm opposite the disc rotation to increase the relative velocity.
U.S. Pat. No. 4,170,783--Tajima illustrates another arrangement wherein a permanent magnet is secured to the end of the stylus arm distant from the pickup stylus and electromagnetic coils are disposed proximate the permanent magnet to influence the position of the magnet through mutual coupling of magnetic fields and thereby influence the stylus-disc relative velocity responsive to correction signals.
The present inventor discovered that improper alignment of the permanent magnet with the stylus arm, and improper positioning and orientation of the permanent magnet with respect to the coils will produce additional disc-stylus velocity errors as a result of the stylus arm "fishpoling". Fishpoling is defined as the condition of flexure wherein the center of a longitudinal member moves in one direction normal to its longitudinal axis and its ends move in the opposite direction. It is believed that the motion is excited by off axis force components imparted by the transducer coils to the magnet secured to the stylus arm. If the stylus tip is constrained to remain on the disc (e.g., by a leafspring flylead) the stylus pickup electrode on one facet thereof tends to move forward along the information track as the stylus holder tilts back, i.e., as the stylus arm bows toward the disc record, and tends to move backward along the track as the stylus arm bows upward away from the disc record. The stylus-disc velocity errors due to this flexure become particularly accentuated if the stylus arm is excited to bow or flex in one of its resonant modes, causing the stylus translation to become exaggerated. Higher harmonics of the resonant flexure have been observed as well. The stylus positional response of the stylus arm first harmonic is antiphase with the fundamental response while the second harmonic response is in phase. Harmonics above the second generally require stimulus at a frequency too high to be of concern. The magnitude of the resonant modes depends on details of the stylus arm structure, particularly the arm shaft diameter, length and mass. One particular hollow aluminum stylus arm 0.1 cm in diameter and 6.25 cm long consistently indicated a fundamental mode occurring at a stimulating frequency of 250-300 Hz, a first harmonic at 500-600 Hz and a further harmonic or peak between 900-1500 Hz.
Careful alignment of the permanent magnet with the stylus, so that off axis force components are substantially eliminated, significantly reduces stylus arm flexure or fishpoling. One method of attaining such alignment is to form the permanent magnet with a hole therethrough for coaxial mounting directly to the stylus arm. Unfortunately, as the stylus translates normal or parallel to the disc surface respectively due to disc warp or track eccentricity, etc., the stylus arm can pivot about its supporting elements tending to misalign the permanent magnet with its driving coils creating an off axis force component and thereby creating a stimulus for fishpoling.